Group III-V nitride compound semiconductor laser devices illustrated to date that have operated for prolonged periods of time, particularly as reported by Shuji Nakamura and his coworkers, have employed AlGaN. An example of the growth of AlGaN layers is exemplified in the U.S. Pat. No. 5,290,393 to Nakamura. Such AlGaN layers are grown in MOCVD at temperatures between 900.degree. C. and 1200.degree. C. Upon heating and cooling sequences in the MOCVD fabrication of devices, the as-grown layers are subjected to tremendous stresses since the AlGaN layers do not expand or contract at the same rate as the sapphire substrate, their thermal expansion coefficients being so different. As a result, large and small micro-cracks form in such as-grown layers that spread from the sapphire substrate upward through the layers. While the mechanisms causing these cracks are not fully understood, they are likely caused by the thermal expansion coefficient mismatch between the as-grown materials which do not expand or contract at the same rate as the substrate. It was also repeatedly shown that there are two factors severely aggravating cracking in nitride-based structures: presence of thick (e.g. &gt;0.2 .mu.m) layers of AlGaN and/or doping with both p- and n-type impurities. The higher temperature heating of the substrate during AlGaN layer growth and the subsequent cooldown places the as-grown layers on the substrate under mechanical and lattice stresses upon their contraction at room temperature.
In a Group III-V nitride laser device, the structure may be of the so-called separate confinement type, such as, comprising an InGaN active layer or quantum well or a multiple quantum well region of InGaN quantum wells with InGaN or GaN barriers, cladded between GaN waveguide (or core) layers which in turn are cladded by confinement layers of p-type and n-type AlGaN. The general requirement of the separate confinement structure: step-like increase of band gap and corresponding decrease in refractive index from the active layer through waveguide layers to confinement (or cladding layers) is satisfied in this case.
As Al is added to or increased in Al.sub.x Ga.sub.1-x N, the growth temperature for growing the layer has to be correspondingly increased. Because of these high temperatures, diffusion can occur within the InGaN active region. InGaN is grown at much lower temperatures, such as in the range of about 600.degree. C. to about 800.degree. C. These higher growth temperatures for AlGaN, such as in the case of growing the upper p-type AlGaN layer at 1,000.degree. C. or more, will heat up the InGaN layer or layers and can induce atomic rearrangement in these layers. The indium can start clustering at Group III lattice sites through the process of elemental interdiffusion. While not well understood, it is believed that when the InGaN layer is initially grown, the In and Ga atoms which are randomly distributed on the Group III lattice sites, forming a homogenous alloy. When the InGaN layer is subjected later to higher temperatures, particularly well above its growth temperature range, there is an exchange and redistribution of In and Ga atoms at the Group III sites. It could be an equilibrium condition comprising an InN-rich region and a GaN-rich region. Equilibrium is suppressed by growing the InGaN layer at low temperatures so that a homogeneous mix of In and Ga in the lattice structure is achieved. In any case, when the InGaN layer or layers are heated to temperatures in excess of its growth temperature range, InN-rich and GaN-rich clusters can form in the as-grown InGaN layer or layers, causing their desired optical properties to be substantially deteriorated.
In spite of these above mentioned problems, AlGaN layers are the present choice for blue LED and laser devices because they can provide both good electrical and optical confinement particularly if the above described problems can be overcome on a regular high-yield basis.
It is an object of this invention to provide a Group III-V nitride compound semiconductor light emitting device, such as Group III-V nitride lasers and LED's, that eliminates the foregoing problems by avoiding the use of homogeneous AlGaN or the extensive use of aluminum nitride in the fabrication of these types of devices and provide cladding layers that are designed with the objective of suppressing interlayer cracking.